clissold345 wrote:hiriskpaul wrote:
The second reason is that LFP cannot simply be put in instead of lead acid, even though the voltages are similar. The way LFP is charged is quite different to lead acid and the charging system will need to be upgraded to avoid damaging LFP. This is particularly a problem if charging off an alternator and the alternator itself can be damaged if used to directly charge LFP due to the very low internal resistance of LFP and/or the LFP suddenly disconnecting when the BMS judges the cells are full.
This is not true. LiFePO4 batteries can basically be dropped in in place of lead acid batteries. There's a 12V boating group on facebook (administered by Phil Brooke-Little), where this has been discussed for 2 or 3 years. If you search around you can find all the details there.
Everything I have read on yachting forums, with links to RV experiences, says you should not simply replace lead acid with LFPs. Definitely not for engine starting as LFP cannot deliver the required cold cranking amps. For domestic use they are fine, but as I have mentioned, they have different charging requirements. That's why modern 12V chargers have a lithium or LiFePO4 setting. In particular, LFP batteries should not be left on float, but lead acid should. When fully charged, the LFP charging should stop. Charging LFP straight from an alternator will work but there is a risk that the BMS will suddenly disconnect to prevent overcharging. This can produce a high back EMF which may fry your alternator diodes. Another risk is that when running at low speeds the alternator may overheat as the LFP internal resistance is so low. It is like shorting your alternator. There are some examples of smoking alternators on YouTube.
A recommended way to avoid the issues, which I intend to follow, is to connect a battery to battery charger (eg Victron) between the alternator and the LFP bank. The alternator is also connected directly to a lead acid starter battery. This setup ensures any spikes from the alternator are absorbed harmlessly by the starter battery.
There are other ways of handling the issues, but everything I have seen says you should not just drop in LFP batteries where you used to have lead.
I will check out the Facebook group though. Have you just dropped in LFPs where you had lead acid without changing anything?
ps, this site is useful
https://batteryuniversity.com/